Blood Alcohol
Content Calculator
Estimate your BAC using the Widmark formula — the same method used by forensic scientists and medical professionals. Know your number before you make a decision.
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Time to Sober (0.00%)
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Understanding BAC and Why It Matters
Blood Alcohol Content (BAC) is the concentration of alcohol in your bloodstream, expressed as a percentage. A BAC of 0.08% — the legal driving limit in the US, UK, and most of Europe — means there are 0.08 grams of alcohol per 100 millilitres of blood. It sounds small, but at that level, your reaction time is significantly slower, your judgment is impaired, and your risk of a crash is roughly four times higher than at 0.00%.
The problem with alcohol is that it's uniquely deceptive. Unlike pain, which gets worse the more you push through it, impairment from alcohol often feels less severe than it actually is — because the part of your brain that would normally register the problem is itself impaired. That's why objective tools like a BAC calculator exist: to give you a number that doesn't lie, even when your perception does.
The Widmark Formula Explained
This calculator uses the Widmark formula, developed by Swedish physician Erik Widmark in the 1930s and still used by forensic scientists and medical professionals today. It's the most widely validated method for estimating BAC from known alcohol intake.
A = grams of alcohol consumed
W = body weight in lbs
r = 0.73 (male) or 0.66 (female)
H = hours since first drink
The r value (Widmark factor) reflects the difference in body water distribution between males and females. Women generally have a higher proportion of body fat relative to body water than men of the same weight, which means alcohol concentrates more in a smaller water volume, resulting in a higher BAC from the same intake.
The 0.015 subtraction per hour reflects the average metabolic elimination rate — the liver processes roughly 0.015% BAC worth of alcohol per hour in a healthy adult. This rate doesn't change based on how much coffee you drink, how much water you have, or how many times you've eaten. It's fixed by your liver's enzymatic capacity.
What the Numbers Actually Mean
0.02–0.03% — Most people feel some warmth and mild relaxation. Reaction time is essentially unaffected at this level, but cognitive performance on complex tasks begins to subtly decline.
0.05–0.07% — Noticeable relaxation, reduced inhibitions, some impairment to balance and coordination. Many people feel confident and social — but that confidence is itself a sign of reduced judgment. You may feel fine to drive. You are not.
0.08% — The legal driving limit in most countries. At this level, the evidence is unambiguous: reaction time is measurably slower, divided attention tasks (exactly what driving requires) are significantly impaired, and the subjective feeling of impairment is often less severe than the objective impairment.
0.10–0.15% — Obvious impairment to most observers. Slurred speech, poor coordination, significantly slowed reaction time. Motor control is meaningfully degraded.
0.20%+ — Serious impairment. Risk of vomiting while incapacitated (aspiration risk), blackouts, loss of consciousness. Medical attention may be needed.
0.30%+ — Life-threatening. The respiratory centres of the brain can be suppressed. This is the range where alcohol poisoning becomes acutely dangerous.
Safety Rules — Please Read
- Never drive if your BAC is at or above 0.08% — and be aware that some countries set the limit lower (0.05% in most of Europe, 0.02% for new drivers in the UK).
- This calculator is an estimate, not a breathalyser. Individual metabolism varies. Factors like food consumption, medication, liver health, and fatigue all affect true BAC in ways this formula cannot account for.
- Coffee, water, and fresh air do not sober you up. Only time does. The 0.015%/hr elimination rate is fixed regardless of what you consume.
- If someone is unconscious or unresponsive after drinking, place them in the recovery position and call emergency services immediately.
- If you're unsure — don't drive. Arrange a taxi, a designated driver, or stay where you are.
Factors That Affect Your Actual BAC
The Widmark formula gives a solid estimate, but real-world BAC is influenced by several factors beyond what any formula can capture. Food is the biggest one — a meal before or during drinking slows alcohol absorption significantly, meaning peak BAC is lower and reached more slowly. An empty stomach does the opposite.
Medication can interact with alcohol in ways that amplify impairment well below the BAC threshold. Antihistamines, benzodiazepines, antidepressants, and many painkillers all have documented interactions. If you're on regular medication, consult your prescriber about alcohol.
Tolerance affects how impaired you feel at a given BAC, but it doesn't meaningfully change your actual BAC or your driving ability. High-tolerance drinkers often believe they're safe to drive at a BAC where they're statistically at significant crash risk — the impairment is real even if they feel functional.
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Frequently Asked Questions
Your body eliminates roughly 0.015% BAC per hour — a fixed rate determined by your liver's enzymatic capacity, not by how much you drink or what you do afterward. To estimate your sober time, simply divide your peak BAC by 0.015. At 0.12% BAC, that's 8 hours to fully sober — which explains why many people who stop drinking at midnight are still over the limit the next morning.
Food doesn't lower your BAC once alcohol is already in your bloodstream — but eating before or during drinking significantly slows the rate of absorption, which means alcohol enters your blood more slowly and your peak BAC is lower than it would be on an empty stomach. A full meal can reduce peak BAC by 30–50% compared to drinking on an empty stomach. Once the alcohol is absorbed, only time eliminates it.
Two main reasons. First, women typically have a higher ratio of body fat to body water than men of the same weight — and since alcohol distributes into body water (not fat), it becomes more concentrated. Second, women generally have lower levels of alcohol dehydrogenase, the stomach enzyme that begins breaking down alcohol before it reaches the bloodstream. This means more alcohol is absorbed per drink. The Widmark formula accounts for this through the gender factor (r = 0.73 for men, 0.66 for women).
In the US, one standard drink contains 14 grams of pure alcohol — equivalent to 12 oz regular beer (5% ABV), 5 oz wine (12% ABV), or 1.5 oz spirits (40% ABV). In the UK, a unit is slightly smaller at 8 grams. Be aware that many pub pours, cocktails, and home measures exceed the "standard" definition — a large glass of wine at a restaurant is often closer to 2–2.5 standard drinks, not one.
Disclaimer: This BAC calculator provides estimates only and cannot replace a professional breathalyser or blood test. Do not use this tool to make decisions about driving or operating machinery. Individual results vary significantly based on factors this formula cannot account for. If in doubt, do not drive.